The present invention relates to a digital waveform shaping circuit capable of accurately reconstructing a deteriorated digital signal.
Digital signals are recorded in high density and reproduced with PCM (pulse-code modulation) recorders or the like. In this case, because of intersymbol interference between recording bit patterns, the quality of reproduced signals is poor and their demodulation difficult. This problem is especially common in recording-reproducing systems using magnetic tape and heads, and having a narrow range.
With commercially available PCM encoders-decoders, for example, audio signals are recorded and reproduced by the PCM system, using a commercially available video cassette system. A television synchronizing-signal is recorded between PCM audio signals so that the PCM audio signals before and after the synchro-signal are subjected to the intersymbol interference, deteriorating their properties. Particularly, the recording density of the commercially available video cassette system has been made high, which makes it difficult to arrange the recording-reproducing range sufficiently wide. This makes it difficult to correctly demodulate the system signals. When a threshold level is conventionally held constant at a comparator at the time of reproducing and wave-shaping digital signals recorded on a recording medium, the recorded signal waveform peak may be too low to reach the threshold level.
Referring to the waveforms shown in FIGS. 1A to 1D, a signal to be demodulated as the signal shown in FIG. 1C is incorrectly demodulated as the signal shown in FIG. 1D when a deteriorated analog signal Z synchronized to the synchronizing signal shown in FIG. 1B is detected at the rise of its synchronizing signal, while the threshold level is held fixed at the position shown by the broken line in FIG. 1A.